As Joe says, find a job that doesn't require overtime. It may even be the company that you are working for. Once you decided for yourself that you are willing to leave this company and work somewhere else, and once you have found that there are other jobs around, lots of pressure is gone from you and you are in a much better position.
You can for example demand that you should be paid for your overtime. In most places, the company must pay you for overtime. An exception is the USA where you can be "exempted" and have to do unpaid overtime if required by the business. However, a business not hiring enough people to do all the work means it is not required by the business. You will likely see how suddenly overtime isn't required anymore if it isn't free for the company. Alternatively, you will see how overtime becomes much more acceptable if it puts money in your pocket.
OR you can tell them that your overtime means you are constantly overworked and not able to be as productive as you should be, and that you would do a much better job without the overtime. Which any decent manager would know anyway. If they need you to do overtime, then they are not really in a position to fire you, right? If they threaten to fire you, that's also a very strong argument. And knowing that you can get another job means you can make all these arguments with confidence.